trouble with expansion in ltproperties

Question

I am struggling to control expansion in code recorded with ltproperties. In the following example, I am trying to record the body of an environment as stated, without expansion (so as if with \exp_not:n). Of course, to work with ltproperties it needs to first be stored in a variable, so I use \exp_not:V on the variable in the fourth argument of \property_new:nnnn.

\documentclass{article}
\ExplSyntaxOn
\tl_new:N \l__recordenv_contents_tl
\property_new:nnnn { recordenv/contents } { now } { }
  { \exp_not:V \l__recordenv_contents_tl }
\NewDocumentEnvironment{recordenv}{ m +b }
  {
    \tl_set:Nn \l__recordenv_contents_tl { #2 }
    \tl_show:N \l__recordenv_contents_tl
    \property_record:nn { #1-recordenv } { recordenv/contents }
  }
  {}
\ExplSyntaxOff
\begin{document}
\section{First section}
\begin{recordenv}{foo}
some text and \thesection
\end{recordenv}
\section{Second section}
\end{document}

If I check the log, the tl variable contains what I want:

> \l__recordenv_contents_tl=some text and \thesection

However the aux file contains

\relax 
\@writefile{toc}{\contentsline {section}{\numberline {1}First section}{1}{}\protected@file@percent }
\new@label@record{foo-recordenv}{{recordenv/contents}{some text and 2}}
\@writefile{toc}{\contentsline {section}{\numberline {2}Second section}{1}{}\protected@file@percent }
\gdef \@abspage@last{1}

So when written to the aux, the body is still being expanded since \thesection becomes a number. Furthermore, this expansion is happening at shipout since the number \thesection expands to is 2, not 1 as you'd expect since the property was set with now.

Now, I thought maybe this was the same issue as in this question I asked about expansion in \addtocontents. There the issue was that two separate expansions were happening so adding an extra \exp_not:n was necessary. However, \exp_not:n { \exp_not:V \l__recordenv_contents_tl } just leads to an empty argument in the aux so I assume that's not what's happening here. I also tried \exp_not:o { \l__recordenv_contents_tl } but the effect is the same as with \exp_not:V.

My questions are

1. why is there expansion happening on the tl variable after being V-expanded, and
2. why is this second expansion happening at shipout?

Added

I think I now understand why \exp_not:V alone can't work the way I want, and why \exp_not:n { \exp_not:V <tlvar> } fails. The content is expanded once at definition, stripping the \exp_not:n, then expanded again when writing to the file during shipout at which time <tlvar> is empty. Is this correct? Looking at the ltproperties code, I am still confused why \iow_shipout_x:Nx is used in \property_record:nn for all properties, even those with setpoint now.

Answer 1

LaTeX writes on the .aux file with \protected@write that performs full expansion (but preserves commands declared robust).

You need to protect the contents from expansion at a different level.

\documentclass{article}
\ExplSyntaxOn
\tl_new:N \l__recordenv_contents_tl
\property_new:nnnn { recordenv/contents } { now } { }
  { \exp_not:V \l__recordenv_contents_tl }
\NewDocumentEnvironment{recordenv}{ m +b }
  {
    \tl_set:Nn \l__recordenv_contents_tl { \exp_not:n { #2 } }
    %\tl_show:N \l__recordenv_contents_tl
    \property_record:nn { #1-recordenv } { recordenv/contents }
  }
  {}
\ExplSyntaxOff
\begin{document}
\section{First section}
\begin{recordenv}{foo}
some text and \thesection
\end{recordenv}
\section{Second section}
\end{document}

The .aux file

\relax 
\@writefile{toc}{\contentsline {section}{\numberline {1}First section}{1}{}\protected@file@percent }
\new@label@record{foo-recordenv}{{recordenv/contents}{some text and \thesection }}
\@writefile{toc}{\contentsline {section}{\numberline {2}Second section}{1}{}\protected@file@percent }
\gdef \@abspage@last{1}